fp-compose.ts

export type UnaryFn<T, R> = (arg: T) => R;

export function compose<R>(fn: UnaryFn<any, R>): UnaryFn<any, R>;
export function compose<T1, R>(fn1: UnaryFn<T1, R>): UnaryFn<T1, R>;
export function compose<T1, T2, R>(
  fn2: UnaryFn<T2, R>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T1, T2, T3, R>(
  fn3: UnaryFn<T3, R>,
  fn2: UnaryFn<T2, T3>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T1, T2, T3, T4, R>(
  fn4: UnaryFn<T4, R>,
  fn3: UnaryFn<T3, T4>,
  fn2: UnaryFn<T2, T3>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T1, T2, T3, T4, T5, R>(
  fn5: UnaryFn<T5, R>,
  fn4: UnaryFn<T4, T5>,
  fn3: UnaryFn<T3, T4>,
  fn2: UnaryFn<T2, T3>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T1, T2, T3, T4, T5, T6, R>(
  fn6: UnaryFn<T6, R>,
  fn5: UnaryFn<T5, T6>,
  fn4: UnaryFn<T4, T5>,
  fn3: UnaryFn<T3, T4>,
  fn2: UnaryFn<T2, T3>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T1, T2, T3, T4, T5, T6, T7, R>(
  fn7: UnaryFn<T7, R>,
  fn6: UnaryFn<T6, T7>,
  fn5: UnaryFn<T5, T6>,
  fn4: UnaryFn<T4, T5>,
  fn3: UnaryFn<T3, T4>,
  fn2: UnaryFn<T2, T3>,
  fn1: UnaryFn<T1, T2>
): UnaryFn<T1, R>;
export function compose<T = any, R = any>(
  ...fns: ((arg: any) => any)[]
): UnaryFn<T, R>;
export function compose(...fns: Function[]): Function {
  return (arg: any) => fns.reduceRight((result, fn) => fn(result), arg);
}